Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree

Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree

[Benjamin Herrenschmidt]

>   The root cause of this bug lies in the fact that the XICS interrupt controller
> uses a radix tree for its reverse irq mapping and that we cannot allocate the tree
> nodes (even GFP_ATOMIC) with preemption disabled.

Is that yet another caes of -rt changing some basic kernel semantics ?

>   In fact, we have 2 nested preemption disabling when we want to allocate
> a new node:
> 
>   - setup_irq() does a spin_lock_irqsave() before calling xics_startup() which
>     then calls irq_radix_revmap() to insert a new node in the tree
> 
>   - irq_radix_revmap() also does a spin_lock_irqsave() (in irq_radix_wrlock())
>     before the radix_tree_insert()
> 
>   The first patch moves the call to irq_radix_revmap() from xics_startup() out to
> xics_host_map_direct() and xics_host_map_lpar() which are called with preemption
> enabled.

I suppose that would work.

>   The second patch is a little more involved in that it takes advantage of
> the concurrent radix tree to simplify the locking requirements and allows
> to allocate a new node outside a preemption disabled section.
> 
>   I just hope I've correctly understood the concurrent radix trees semantic
> and got the (absence of) locking right.

Hrm, that will need some scrutinity.

Thanks for looking at this.

Cheers,
Ben.

Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree

[Sebastien Dugue]

On Thu, 24 Jul 2008 08:17:38 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> 
> >   The root cause of this bug lies in the fact that the XICS interrupt controller
> > uses a radix tree for its reverse irq mapping and that we cannot allocate the tree
> > nodes (even GFP_ATOMIC) with preemption disabled.
> 
> Is that yet another caes of -rt changing some basic kernel semantics ?

  Ahem, not really new: http://lkml.org/lkml/2007/11/12/211

> 
> >   In fact, we have 2 nested preemption disabling when we want to allocate
> > a new node:
> > 
> >   - setup_irq() does a spin_lock_irqsave() before calling xics_startup() which
> >     then calls irq_radix_revmap() to insert a new node in the tree
> > 
> >   - irq_radix_revmap() also does a spin_lock_irqsave() (in irq_radix_wrlock())
> >     before the radix_tree_insert()
> > 
> >   The first patch moves the call to irq_radix_revmap() from xics_startup() out to
> > xics_host_map_direct() and xics_host_map_lpar() which are called with preemption
> > enabled.
> 
> I suppose that would work.

  It should indeed. Instead of inserting the new mapping at request_irq() time,
we do it a bit before at create_irq_mapping time.

> 
> >   The second patch is a little more involved in that it takes advantage of
> > the concurrent radix tree to simplify the locking requirements and allows
> > to allocate a new node outside a preemption disabled section.
> > 
> >   I just hope I've correctly understood the concurrent radix trees semantic
> > and got the (absence of) locking right.
> 
> Hrm, that will need some scrutinity.

  Yep, that will need a few more pair of eyes along with brains behind those ;-)

  Thanks,

  Sebastien.

> 
> Thanks for looking at this.
> 
> Cheers,
> Ben.
> 
> 
>